The present invention relates to the field of automatic exposure control systems for industrial cameras.
Exposure control systems are known in the art which generate a series of pulses, wherein the pulses thereof have intervals which vary as of function of an input parameter such as the intensity of illumination of the exposure lamps. If longer exposure intervals are desired, the duration of the pulses are increased so that when such pulses are applied to a counter and are counted down, it takes a longer period for the count to reach a predetermined value to define an exposure interval. Conversely if the duration of the pulses are reduced, frequency of counting down increases to thus define a shorter exposure interval.
For example, in U.S. Pat. No. 3,795,444 to Glidden et al, the input parameter for controlling a relaxation oscillator is generated by photodiode 20 which is light coupled to exposure lamp 17. Changes in the intensity of lamp 17 change the magnitude of the current applied to integrating capacitor 43 to vary the slope of the saw tooth pulses generated at the output circuit of amplifier 40. Thus changes in the illumination intensity of lamp 17 produce changes in the width of the pulses produced at the output of comparator 52 which in turn produce corresponding changes in the time required for counter 80 to reach a predetermined count to in turn define the exposure interval. A significant drawback of this system is that the pulse widths of the pulses applied to the counter vary non-linearly with respect to the light intensity of lamp 17 since such widths are a reciprocal function of the strength of the current applied by photodiode 20 to integrating capacitor 43.
Furthermore, the Glidden patent does not produce an exposure interval which is a linear function of changes in magnification of the image, which is an object of the present invention.